Enhanced Surface Reaction Kinetics and Charge Separation of p–n Heterojunction Co3O4/BiVO4 Photoanodes

Single-crystal silicon-based electrodes for unbiased solar water splitting: current status and prospects

This review describes recent developments of single-crystal silicon (Si) as the photoelectrode material for solar water splitting, including the promising strategies to obtain highly efficient and stable single-crystal Si-based photoelectrodes for hydrogen evolution and water oxidation, as well as the future development of spontaneous solar water splitting with single-crystal Si-based tandem cells.

Novel hierarchical Sn3O4/BiOX (X = Cl, Br, I) p–n heterostructures with enhanced photocatalytic activity under simulated solar light irradiation

Sn₃O₄/BiOX (X = Cl, Br, I), a series of p–n-heterojunction-based photocatalysts, were prepared by a combination of an ultrasonic-assisted precipitation–deposition method and hydrothermal method.

Enhanced Photoelectrochemical Water Oxidation Performance on BiVO4 by Coupling of CoMoO4 as a Hole-Transfer and Conversion Cocatalyst

Manipulation of interfacial charge separation and transfer is one of the primary breakthroughs to improve the water oxidation activity and stability of BiVO₄ photoanode. In the present work, a CoMoO₄-coupled BiVO₄ (BiVO₄/CoMoO₄) film was designed and prepared as the photoanode for photoelectrochemical (PEC) water oxidation. Compared with the bare BiVO₄ film, obviously improved PEC water oxidation performance was observed on the BiVO₄/CoMoO₄ film. Specifically, a higher water oxidation photocurrent density of 3.04 mA/cm² at 1.23 V versus RHE was achieved on the BiVO₄/CoMoO₄ photoanode, which is of about 220% improvement over bare BiVO₄ photoanode (1.34 mA/cm² at 1.23 V vs RHE). In addition, the BiVO₄/CoMoO₄ film photoanode was of better stability and faster hole-to-oxygen kinetics for water oxidation, without significant activity attenuation for 6 h of reaction at 0.65 V versus RHE. The enhanced water oxidation performance on the BiVO₄/CoMoO₄ film photoanode can be ascribed to the synergistic effect of the following factors: (i) thermodynamically, the photogenerated holes of BiVO₄ are directionally transferred to CoMoO₄ through their physical coupling interface and valance band potential matching; and (ii) kinetically, the transferred holes induce the formation of Co³⁺-active sites on CoMoO₄ that could synergistically oxidize H₂O to molecular O₂ with stable activity.

Boosted Water Oxidation Activity and Kinetics on BiVO4 Photoanodes with Multihigh-Index Crystal Facets

The crystal facet of the BiVO₄ photoanode has potential influence on its charge-transfer and separation properties as well as water oxidation kinetics. In the present work, a BiVO₄ polyhedral film with exposed {121}, {132}, {211}, and {251} high-index facets was synthesized by a facile Bi₂O₃ template-induced method and investigated as a photoanode for water oxidation. In comparison with the normal BiVO₄ film with a {121} monohigh-index facet, the BiVO₄ film with multihigh-index crystal facets shows higher activity and faster kinetics for photoelectrochemical water oxidation. Specifically, a higher photocurrent density of 1.21 mA/cm² was achieved on the multihigh-index facet BiVO₄ photoanode at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 M Na₂SO₄, which is about 200% improved over the normal BiVO₄ photoanode (0.61 mA/cm² at 1.23 V vs RHE). In addition, a negative shift of 300 mV onset potential for water oxidation was observed on the as-prepared BiVO₄ photoanode (0.22 V vs RHE) relative to the normal BiVO₄ photoanode (0.52 V vs RHE) in 0.1 M Na₂SO₄. Although the UV-vis absorbance property and water oxidation pathway not be changed, the charge-transfer and separation properties as well as the overall water oxidation kinetics on the multihigh-index facet BiVO₄ film were boosted obviously. Theory calculations reveal that the adsorption of H₂O molecules on BiVO₄{121} and {132} high-index facets is energetically favorable for subsequent dissociation and oxidation relative to that on {010} and {110} low-index facets. Furthermore, the water oxidation limiting step on {121} and {132} high-index facets of BiVO₄ is changed to the step of two protons reacting with •O to form •OOH species (•O + H₂O_(l) + 2H⁺ + 2e^- → •OOH + 3H⁺ + 3e^-), which is different from the limiting step on {010} and {110} low-index facets that corresponds to the dissociation of H₂O to •OH (2H₂O_(l) + • → •OH + H₂O_(l) + H⁺ + e^-). In addition, the overpotential of water oxidation limiting step on BiVO₄{121} and {132} high-index facets is lower than that on {010} and {110} low-index facets.

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes

Single-source precursors are used to produce nanostructured BiVO₄ photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO₄ films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO₄ films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm² Co-BiVO₄ photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.

Disentangling the Role of Surface Chemical Interactions on Interfacial Charge Transport at BiVO4 Photoanodes

Chemical transformations that occur on photoactive materials, such as photoelectrochemical water splitting, are strongly influenced by the surface properties as well as by the surrounding environment. Herein, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge-carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on bismuth vanadate. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron-trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge-carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding functional characteristics of water splitting photoanodes, devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.

Ferrihydrite-Modified Ti-Fe2 O3 as an Effective Photoanode: The Role of Interface Interactions in Enhancing the Photocatalytic Activity of Water Oxidation

Semiconductor electrodes integrated with cocatalysts are key components of photoelectrochemistry (PEC)-based solar-energy conversion. However, efforts to optimize the PEC device have been limited by an inadequate understanding of the interface interactions between the semiconductor-cocatalyst (sem|cat) and cocatalyst-electrolyte (cat|ele) interface. In our work, we used ferrihydrite (Fh)-modified Ti-Fe₂ O₃ as a model to explore the transfer process of photogenerated charge carriers between the Ti-Fe₂ O₃ -Fh (Ti-Fe₂ O₃ |Fh) interface and Fh-electrolyte (Fh|ele) interface. The results demonstrate that the biphasic structure (Fh/Ti-Fe₂ O₃ ) possesses the advantage that the minority hole transfer from Ti-Fe₂ O₃ to Fh is driven by the interfacial electric field at the Ti-Fe₂ O₃ |Fh interface; meanwhile, the holes reached at the surface of Fh can rapidly inject into the electrolyte across the Fh|ele interface. As a benefit from the improved charge transfer at the Ti-Fe₂ O₃ |Fh and Fh|ele interface, the photocurrent density obtained by Fh/Ti-Fe₂ O₃ can reach 2.32 mA cm^-2 at 1.23 V versus RHE, which is three times higher than that of Ti-Fe₂ O₃ .

High Pressure Photoreduction of CO2: Effect of Catalyst Formulation, Hole Scavenger Addition and Operating Conditions

The photoreduction of CO2 is an intriguing process which allows the synthesis of fuels and chemicals. One of the limitations for CO2 photoreduction in the liquid phase is its low solubility in water. This point has been here addressed by designing a fully innovative pressurized photoreactor, allowing operation up to 20 bar and applied to improve the productivity of this very challenging process. The photoreduction of CO2 in the liquid phase was performed using commercial TiO2 (Evonink P25), TiO2 obtained by flame spray pyrolysis (FSP) and gold doped P25 (0.2 wt% Au-P25) in the presence of Na2SO3 as hole scavenger (HS). The different reaction parameters (catalyst concentration, pH and amount of HS) have been addressed. The products in liquid phase were mainly formic acid and formaldehyde. Moreover, for longer reaction time and with total consumption of HS, gas phase products formed (H2 and CO) after accumulation of significant number of organic compounds in the liquid phase, due to their consecutive photoreforming. Enhanced CO2 solubility in water was achieved by adding a base (pH = 12–14). In basic environment, CO2 formed carbonates which further reduced to formaldehyde and formic acid and consequently formed CO/CO2 + H2 in the gas phase through photoreforming. The deposition of small Au nanoparticles (3–5 nm) (NPs) onto TiO2 was found to quantitatively influence the products distribution and increase the selectivity towards gas phase products. Significant energy storage in form of different products has been achieved with respect to literature results.

Rational design and synthesis of highly oriented copper-zinc ferrite QDs/titania NAE nano-heterojunction composites with novel photoelectrochemical and photoelectrocatalytic behaviors

This work reported that novel highly oriented and vertically aligned stoichiometric copper- and zinc-based ferrites, i.e., Cu0.5Zn0.5Fe2O4 quantum dots (QDs) anchored with TiO2 nanotube array electrode (NAE) composites, with n-n nano-heterojunctions and highly effective simulated solar light harvesting could be successfully achieved via electrochemical anodization followed by a vacuum-assisted impregnation strategy. It has been observed that Cu0.5Zn0.5Fe2O4 QDs/TiO2 NAE composites exhibit distinctly enhanced visible light photoelectrocatalytic (PEC) performance toward the degradation of typical pollutants including sulfamethoxazole (SMX) and methylene blue (MB) as compared to that of pristine TiO2 NAEs, which can be attributed to the synergistic effect of heterostructures with strong interfacial interaction and abundant 1D nanotube array structures to facilitate efficient spatial charge separation and interfacial transfers. The cocatalyst-anchoring of ternary oxides with derived spinel crystal structures onto nanotube arrays forming novel nanocomposites have obviously achieved remarkably enhanced photoelectrochemical (PE) conversion efficiencies, up to a dedicated value of 3.75%, under visible light irradiation as compared to that of 0.88% for aligned standalone TiO2 NAEs. Transient absorption spectroscopy quantitatively indicated long-lived photo-holes with lifetimes exceeding 72.23 μs generated among Cu0.5Zn0.5Fe2O4 QDs/TiO2 NAE nanocomposites. Electron spinning resonance (ESR) demonstrated that more ˙O2- species derived from molecular uptake played the predominant role in the PEC oxidations of SMX and MB species. Moreover, the binding energy of the onset edge (Evf) and Fermi level (Ef) of Cu0.5Zn0.5Fe2O4 QDs/TiO2 NAEs indicated that Cu0.5Zn0.5Fe2O4 QDs modification could considerably enhance the visible light harvesting and adsorption properties of TiO2 NTs. Furthermore, Cu0.5Zn0.5Fe2O4 QDs/TiO2 NAEs achieved up to 50% PEC degradation efficiency and 52.4% COD removal with regard to practical textile wastewater when irradiated by simulated sunlight. This work has provided new insights into the molecular tailing and coupling of multiple spinels with TiO2 NTs possessing remarkable visible light harvesting and sensitization characteristics, which would offer a prospective strategy toward designing highly efficient and easily recyclable photocatalytic materials for environmental remediation and solar energy utilizations and conversions both simultaneously and standalone.

Ultrathin Fe-NiO nanosheets as catalytic charge reservoirs for a planar Mo-doped BiVO4 photoanode

Charge accumulation at the interface reflects the charge separation and recombination kinetics, and will strongly contribute to the photoelectrochemical reactions.

The energy conversion efficiency of a photoelectrochemical system is intimately connected to a number of processes, including light absorption, charge excitation, separation and transfer processes. Of these processes, the charge transfer rate at the electrode|electrolyte interface is the slowest and, hence, the rate-limiting step causing charge accumulation. Such an understanding underpins efforts focused on applying highly active electrocatalysts, which may contribute to the overall performance by augmenting surface charge accumulation, prolonging charge lifetime or facilitating charge transfer. How the overall effect depends on these individual possible mechanisms has been difficult to study previously. Aiming at advancing knowledge about this important interface, we applied first-order serial reactions to elucidate the charge excitation, separation and recombination kinetics on the semiconductor|electrocatalyst interfaces in air. The study platform for the present work was prepared using a two-step Mo-doped BiVO₄ film modified with an ultrathin Fe-doped NiO nanosheet, which was derived from an Fe-doped α-Ni(OH)₂ nanosheet by a convenient precipitation and ion-exchange method. The simulation results of the transient surface photovoltage (TSPV) data showed that the surface charge accumulation was significantly enhanced, even at an extremely low coverage (0.12–120 ppm) using ultra-thin Fe-NiO nanosheets. Interestingly, no improvement in the charge separation rate constants or reduction of recombination rate constants was observed under our experimental conditions. Instead, the ultra-thin Fe-NiO nanosheets served as a charge storage layer to facilitate the catalytic process for enhanced performance.

Thickness-dependent photoelectrochemical properties of a semitransparent Co3O4 photocathode

Co₃O₄ has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel Co₃O₄ electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co₃O₄ samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent Co₃O₄ samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co₃O₄ related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two Co₃O₄ photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h^-1 cm^-1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h^-1·cm^-1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co₃O₄ photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.

Preparation of nanoporous BiVO4/TiO2/Ti film through electrodeposition for photoelectrochemical water splitting

A nanoporous BiVO4/TiO2/Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C for 2 h. The electrodeposition of BiOI film was carried out at different times (10, 30, 100, 500 and 1000 s) in Bi(NO3)3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO4/TiO2/Ti heterojunction film were examined by using different characterization techniques. UV-vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO4/TiO2/Ti film is lower than that of TiO2/Ti. The photocatalytic efficiency of BiVO4/TiO2/Ti heterojunction film was higher compared to that of the TiO2/Ti film owing to the longer transient decay time for BiVO4/TiO2/Ti film (3.2 s) than that of TiO2/Ti film (0.95 s) in our experiment. The BiVO4/TiO2/Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm-2 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti film (1000 s) under irradiation.

Review on nanoscale Bi-based photocatalysts

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Visible light induced electron transfer from a semiconductor to an insulator enables efficient photocatalytic activity on insulator-based heterojunctions

Photogenerated electrons play a vital role in photocatalysis as they can induce the formation of radicals participating in the reaction or recombine with holes preventing them from the subsequent redox reaction. In this work, we explore an Earth-abundant insulator coupled with a semiconductor and construct insulator-semiconductor heterojunctions to effectively realize the efficient electron transfer from the semiconductor to the insulator and thus the enhanced charge carrier separation on the semiconductor. This result will challenge the traditional opinion that free electrons cannot be transferred onto insulators. Taking the BaCO3 insulator as a case study, the combined experimental and theoretical evidence indicates that the photogenerated electrons from the BiOI semiconductor could transfer directly to the BaCO3 insulator through a preformed electron delivery channel when they are coupled to form BaCO3/BiOI heterojunctions. The potential difference between the Bi layer of BiOI (5.03 eV) and the carbonate layer of BaCO3 (12.37 eV) would drive the transfer of excited electrons from Bi atoms across the energy barrier to the adjacent carbonate layer under visible light irradiation. Consequently, the free electrons on BaCO3 can be utilized to produce the oxidative radicals (˙OH, ˙O2- and 1O2) participating in the photocatalytic oxidation reaction. The in situ FT-IR spectra illustrate that the visible light induced active species in the heterojunctions could react with NO, leading to its oxidation to high valence state intermediates (NO+ and NO2+) first and then conversion to the final product of nitrates. This research offers new perspectives to explore insulator-based photocatalysts and unravel the gas-phase photocatalytic reaction mechanism.

A Cobalt-Based Metal-Organic Framework as Cocatalyst on BiVO4 Photoanode for Enhanced Photoelectrochemical Water Oxidation

A metal-organic framework (MOF)-modified bismuth vanadate (BiVO₄ ) photoanode is fabricated by an ultrathin sheet-induced growth strategy, where ultrathin cobalt oxide sheets act as a metal source for the in situ synthesis of Co-based MOF poly[Co₂ (benzimidazole)₄ ] (denoted [Co₂ (bim)₄ ]) nanoparticles on the surface of BiVO₄ . [Co₂ (bim)₄ ] with small particle size and high dispersion can serve as a promising cocatalyst to accept holes transferred from BiVO₄ and boost surface reaction kinetics for photoelectrochemical (PEC) water oxidation. The photocurrent density of a [Co₂ (bim)₄ ]-modified BiVO₄ photoanode can achieve 3.1 mA cm^-2 under AM 1.5G illumination at 1.23 V versus the reversible hydrogen electrode (RHE), which is better than those of pristine and cobalt-based inorganic materials-modified BiVO₄ photoanodes. [Co₂ (bim)₄ ], with porosity and abundant metal sites, exhibits a high surface charge-separation efficiency (83 % at 1.2 V versus RHE), leading to the enhanced PEC activity. This work will bring new insight into the development of MOF materials as competent cocatalysts for PEC water splitting applications.

Facile synthesis of Sb2S3/MoS2 heterostructure as anode material for sodium-ion batteries

A novel Sb₂S₃/MoS₂ heterostructure in which Sb₂S₃ nanorods are coated with MoS₂ nanosheets to form a core-shell structure has been fabricated via a facile two-step hydrothermal process. The Sb₂S₃/MoS₂ heterostructure utilized as the anode of sodium-ion batteries (SIBs) shows higher capacity, superior rate capability and better cycling performance compared with individual Sb₂S₃ nanorods and MoS₂ nanosheets. Specifically, the Sb₂S₃/MoS₂ electrode shows an initial reversible capacity of 701 mAh g^-1 at a current density of 100 mA g^-1, which then remains at 80.1% of the initial performance after 100 cycles at the same current density. This outstanding electrochemical performance indicates that the Sb₂S₃/MoS₂ heterostructure is a very promising anode material for high-performance SIBs.

Bismuth Vanadate with Electrostatically Anchored 3D Carbon Nitride Nano-networks as Efficient Photoanodes for Water Oxidation

In this study, we report a photoanode consisting of a polymeric/inorganic nanojunction between novel nanostructured 3D C₃ N₄ nano-networks and BiVO₄ substrate. This nanojunction is formed such that 3D C₃ N₄ nano-networks with a positively charged surface are efficiently anchored on the BiVO₄ photoanode with a negatively charged surface. This electrostatic self-assembly can initiate and contribute to an intimate contact at the interfaces, leading to an enhanced photoelectrochemical activity and stability compared with that fabricated by non-electrostatic assembly. The C₃ N₄ nano-network/BiVO₄ photoanode achieved a remarkable photocurrent density of 4.87 mA cm^-2 for water oxidation at 1.23 V (vs. reversible hydrogen electrode) after depositing FeOOH/NiOOH as oxygen-evolution co-catalyst, which is among the highest photocurrent densities reported so far for BiVO₄ -based photoanodes.

Porous ZnO-Coated Co3O4 Nanorod as a High-Energy-Density Supercapacitor Material

Co₃O₄ with a high theoretical capacitance has been widely recognized as a promising electrode material for supercapacitor, but its poor electrical conductivity and stability limit its practical applications. Here, we developed an effective synthetic route to synthesize one-dimensional (1D) porous ZnO/Co₃O₄ heterojunction composites. Benefiting from the heterostructure to promote the charge transfer and protect Co₃O₄ from corrosion and the 1D porous structure to improve ion diffusion and prevent structural collapse in charge and discharge process, the as-prepared ZnO/Co₃O₄ composites exhibited an excellent capacitive performance and good cycling stability. The specific capacitance of the ZnO/Co₃O₄-450 (1135 F g^-1 at 1 A g^-1) was 1.4 times higher than that of Co₃O₄ (814 F g^-1), and the high-rate performance for ZnO/Co₃O₄-450 was 4.9 times better than that of Co₃O₄. Also, approximately 83% of its specific capacitance was retained after 5000 cycles at 10 A g^-1. Most importantly, the as-fabricated asymmetric supercapacitor, with a ZnO/Co₃O₄-450 positive electrode and an activated carbon negative electrode, delivered a prominent energy density of 47.7 W h kg^-1 and a high power density of 7500 W kg^-1. Thus, the ZnO/Co₃O₄ composites could serve as a high-activity material for supercapacitor and the preparation method also offers an attractive strategy to enhance the capacitive performance of Co₃O₄.

Surface, Bulk, and Interface: Rational Design of Hematite Architecture toward Efficient Photo-Electrochemical Water Splitting

Collecting and storing solar energy to hydrogen fuel through a photo-electrochemical (PEC) cell provides a clean and renewable pathway for future energy demands. Having earth-abundance, low biotoxicity, robustness, and an ideal n-type band position, hematite (α-Fe₂ O₃ ), the most common natural form of iron oxide, has occupied the research hotspot for decades. Here, a close look into recent progress of hematite photoanodes for PEC water splitting is provided. Effective approaches are introduced, such as cocatalysts loading and surface passivation layer deposition, to improve the hematite surface reaction in thermodynamics and kinetics. Second, typical methods for enhancing light absorption and accelerating charge transport in hematite bulk are reviewed, concentrating upon doping and nanostructuring. Third, the back contact between hematite and substrate, which affects interface states and electron transfer, is deliberated. In addition, perspectives on the key challenges and future prospects for the development of hematite photoelectrodes for PEC water splitting are given.

Tunable syngas production from photocatalytic CO2 reduction with mitigated charge recombination driven by spatially separated cocatalysts

Photocatalytic CO2 reduction represents a sustainable route to generate syngas (the mixture of CO and H2), which is a key feedstock to produce liquid fuels in industry. Yet this reaction typically suffers from two limitations: unsuitable CO/H2 ratio and serious charge recombination. This paper describes the production of syngas from photocatalytic CO2 reduction with a tunable CO/H2 ratio via adjustment of the components and surface structure of CuPt alloys and construction of a TiO2 mesoporous hollow sphere with spatially separated cocatalysts to promote charge separation. Unlike previously reported cocatalyst-separated hollow structures, we firstly create a reductive outer surface that is suitable for the CO2 reduction reaction. A high evolution rate of 84.2 μmol h-1 g-1 for CO and a desirable CO/H2 ratio of 1 : 2 are achieved. The overall solar energy conversion yield is 0.108%, which is higher than those of traditional oxide and sulfide based catalysts (generally about 0.006-0.042%). Finally, density functional theory calculations and kinetic experiments by replacing H2O with D2O reveal that the enhanced activity is mainly determined by the reduction energy of CO* and can be affected by the stability of COOH*.

Boosting photocatalytic overall water splitting by Co doping into Mn3O4 nanoparticles as oxygen evolution cocatalysts

The effect of cobalt doping into a manganese oxide (tetragonal spinel Mn₃O₄) nanoparticle cocatalyst up to Co/(Co + Mn) = 0.4 (mol/mol) on the activity of photocatalytic water oxidation was studied. Monodisperse ∼10 nm Co_yMn_1-yO (0 ≤y≤ 0.4) nanoparticles were uniformly loaded onto photocatalysts and converted to Co_xMn_3-xO₄ nanoparticles through calcination. 40 mol% cobalt-doped Mn₃O₄ nanoparticle-loaded Rh@Cr₂O₃/SrTiO₃ photocatalyst exhibited 1.8 times-higher overall water splitting activity than that with pure Mn₃O₄ nanoparticles. Investigation on the band structure and electrocatalytic water oxidation activity of Co_xMn_3-xO₄ nanoparticles revealed that the Co doping mainly contributes to the improvement of water oxidation kinetics on the surface of the cocatalyst nanoparticles.

Facile Synthesis of Magnetic Photocatalyst Ag/BiVO₄/Mn1-xZnxFe₂O₄ and Its Highly Visible-Light-Driven Photocatalytic Activity

Ag/BiVO₄/Mn_1-xZn_xFe₂O₄ was synthesized with a dip-calcination in situ synthesis method. This work was hoped to provide a simple method to synthesis three-phase composite. The phase structure, optical properties and magnetic feature were confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometer (XPS), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectrophotometer (UV-vis DRS), and vibrating sample magnetometer (VSM). The photocatalytic activity was investigated by Rhodamine B (RhB) photo-degradation under visible light irradiation. The photo-degradation rate of RhB was 94.0~96.0% after only 60 min photocatalytic reaction under visible light irradiation, revealing that it had an excellent visible-light-induced photocatalytic activity. In the fifth recycle, the degradation rate of Ag/BiVO₄/Mn_1-xZn_xFe₂O₄ still reached to 94.0%. Free radical tunnel experiments confirmed the dominant role of •O₂^- in the photocatalytic process for Ag/BiVO₄/Mn_1-xZn_xFe₂O₄. Most importantly, the mechanism that multifunction Ag could enhance photocatalytic activity was explained in detail.

Phase-segregated NiP x @FeP y O z core@shell nanoparticles: ready-to-use nanocatalysts for electro- and photo-catalytic water oxidation through in situ activation by structural transformation and spontaneous ligand removal

The phase-segregated NiP_x@FeP_yO_z core@shell NPs act as a colloidally stable, ready-to-use, and excellent OER active transition metal phosphide-based catalyst.

The high overpotential of the oxygen evolution reaction is a critical issue to be overcome to realize efficient overall water splitting and enable hydrogen generation powered by sunlight. Homogeneous and stable nanoparticles (NPs) dispersed in solvents are useful as both electrocatalysts and cocatalysts of photocatalysts for the electro- and photo-catalytic oxygen evolution reaction, respectively, through their adsorption on various electrode substrates. Here, phase-segregated NiP_x@FeP_yO_z core@shell NPs are selectively synthesized by the reaction of Fe(CO)₅ with amorphous NiP_x seed-NPs. The NiP_x@FeP_yO_z NPs on conductive substrates exhibit higher electrocatalytic activity in the oxygen evolution reaction than those of other metal phosphide-based catalysts. The NiP_x@FeP_yO_z NPs can also be used as a cocatalyst of an anodic BiVO₄ photocatalyst to boost the photocatalytic water oxidation reaction. The excellent catalytic activity and high stability of the NiP_x@FeP_yO_z NPs without any post-treatments are derived from in situ activation through both the structural transformation of NiP_x@FeP_yO_z into mixed hydroxide species, (Ni, Fe)O_xH_y, and the spontaneous removal of the insulating organic ligands from NPs to form a smooth and robust (Ni, Fe)O_xH_y/substrate heterointerface during the oxygen evolution reaction.

Mesoporous Hollow Sb/ZnS@C Core-Shell Heterostructures as Anodes for High-Performance Sodium-Ion Batteries

Combining the advantage of metal, metal sulfide, and carbon, mesoporous hollow core-shell Sb/ZnS@C hybrid heterostructures composed of Sb/ZnS inner core and carbon outer shell are rationally designed based on a robust template of ZnS nanosphere, as anodes for high-performance sodium-ion batteries (SIBs). A partial cation exchange reaction based on the solubility difference between Sb₂ S₃ and ZnS can transform mesoporous ZnS to Sb₂ S₃ /ZnS heterostructure. To get a stable structure, a thin contiguous resorcinol-formaldehyde (RF) layer is introduced on the surface of Sb₂ S₃ /ZnS heterostructure. The effectively protective carbon layer from RF can be designed as the reducing agent to convert Sb₂ S₃ to metallic Sb to obtain core-shell Sb/ZnS@C hybrid heterostructures. Simultaneously, the carbon outer shell is beneficial to the charge transfer kinetics, and can maintain the structure stability during the repeated sodiation/desodiation process. Owing to its unique stable architecture and synergistic effects between the components, the core-shell porous Sb/ZnS@C hybrid heterostructure SIB anode shows a high reversible capacity, good rate capability, and excellent cycling stability by turning the optimized voltage range. This novel strategy to prepare carbon-layer-protected metal/metal sulfide core-shell heterostructure can be further extended to design other novel nanostructured systems for high-performance energy storage devices.

Improving the Photo-Oxidative Performance of Bi2MoO6 by Harnessing the Synergy between Spatial Charge Separation and Rational Co-Catalyst Deposition

It has been reported that photogenerated electrons and holes can be directed toward specific crystal facets of a semiconductor particle, which is believed to arise from the differences in their surface electronic structures, suggesting that different facets can act as either photoreduction or photo-oxidation sites. This study examines the propensity for this effect to occur in faceted, plate-like bismuth molybdate (Bi₂MoO₆), which is a useful photocatalyst for water oxidation. Photoexcited electrons and holes are shown to be spatially separated toward the {100} and {001}/{010} facets of Bi₂MoO₆, respectively, by facet-dependent photodeposition of noble metals (Pt, Au, and Ag) and metal oxides (PbO₂, MnO _x, and CoO _x). Theoretical calculations revealed that differences in energy levels between the conduction bands and valence bands of the {100} and {001}/{010} facets can contribute to electrons and holes being drawn to different surfaces of the plate-like Bi₂MoO₆. Utilizing this knowledge, the photo-oxidative capability of Bi₂MoO₆ was improved by adding an efficient water oxidation co-catalyst, CoO _x, to the system, whereby the extent of enhancement was shown to be governed by the co-catalyst location. A greater oxygen evolution occurred when CoO _x was selectively deposited on the hole-rich {001}/{010} facets of Bi₂MoO₆ compared to when CoO _x was randomly located across all of the facets. The elevated performance exhibited for the selectively loaded CoO _x/Bi₂MoO₆ was ascribed to the greater opportunity for hole trapping by the co-catalyst being accentuated over other potentially detrimental effects, such as the co-catalyst acting as a recombination medium and/or covering reactive sites. The results indicate that harnessing the synergy between the spatial charge separation and the co-catalyst location on the appropriate facets of plate-like Bi₂MoO₆ can promote its photocatalytic activity.

Quantitative hole collection for photoelectrochemical water oxidation with CuWO4

The hole collection efficiency of water oxidation was evaluated for CuWO₄ electrodes from comparisons of the photocurrent of H₂O₂ and Na₂SO₃ oxidation as well as intensity modulated photocurrent spectroscopy (IMPS) measurements. We found current multiplication using H₂O₂, however use of Na₂SO₃ and IMPS revealed quantitative water oxidation at 1.23 V vs. RHE.

New Insights into Mn1-xZnxFe₂O₄ via Fabricating Magnetic Photocatalyst Material BiVO₄/Mn1-xZnxFe₂O₄

BiVO₄/Mn_1-xZn_xFe₂O₄ was prepared by the impregnation roasting method. XRD (X-ray Diffractometer) tests showed that the prepared BiVO₄ is monoclinic crystal, and the introduction of Mn_1-xZn_xFe₂O₄ does not change the crystal structure of BiVO₄. The introduction of a soft-magnetic material, Mn_1-xZn_xFe₂O₄, was beneficial to the composite photocatalyst's separation from the liquid solution using an extra magnet after use. UV-vis spectra analysis indicated that Mn_1-xZn_xFe₂O₄ enhanced the absorption intensity of visible light for BiVO₄. EIS (electrochemical impedance spectroscopy) investigation revealed that the introduction of Mn_1-xZn_xFe₂O₄ enhanced the conductivity of BiVO₄, further decreasing its electron transfer impedance. The photocatalytic efficiency of BiVO₄/Mn_1-xZn_xFe₂O₄ was higher than that of pure BiVO₄. In other words, Mn_1-xZn_xFe₂O₄ could enhance the photocatalytic reaction rate.

Smoothing Surface Trapping States in 3D Coral-Like CoOOH-Wrapped-BiVO4 for Efficient Photoelectrochemical Water Oxidation

Highly efficient oxygen evolution driven by abundant sunlight is a key to realize overall water splitting for large-scale conversion of renewable energy. Here, we report a strategy for the interfacial atomic and electronic coupling of layered CoOOH and BiVO₄ to deactivate the surface trapping states and suppress the charge-carrier recombination for high photoelectrochemical (PEC) water oxidation activity. The successful synthesis of a 3D ultrathin-CoOOH-overlayer-coated coral-like BiVO₄ photoanode effectively tailors the migration route of photocarriers on the semiconductor/liquid interface to realize a great increase of ∼200% in the photovoltage relative to bare BiVO₄, consequently decreasing the corresponding onset potential of PEC water splitting from 0.60 to 0.20 V_RHE. As a result, the unique CoOOH/BiVO₄ photoanode could efficiently perform PEC water oxidation in a neutral aqueous solution (pH = 7) with a high photocurrent density of 4.0 mA/cm² at 1.23 V_RHE and a prominent quantum efficiency of 65% at 450 nm. Electronic structural characterizations and theoretical calculations reveal that the combination of layered CoOOH and BiVO₄ forming interfacial oxo-bridge bonding could greatly eliminate surface trapping states and promote the direct transfer of photogenerated holes from the valence band to the surface water redox potential for water oxidation.

Highly reversible and fast sodium storage boosted by improved interfacial and surface charge transfer derived from the synergistic effect of heterostructures and pseudocapacitance in SnO2-based anodes

Sodium-ion batteries have attracted worldwide attention as potential alternatives for large scale stationary energy storage due to the rich reserves and low cost of sodium resources. However, the practical application of sodium-ion batteries is restricted by unsatisfying capacity and poor rate capability. Herein, a novel mechanism of improving both interfacial and surface charge transfer is proposed by fabricating a graphene oxide/SnO₂/Co₃O₄ nanocomposite through a simple hydrothermal method. The formation of heterostructures between ultrafine SnO₂ and Co₃O₄ could enhance the charge transfer of interfaces owing to the internal electric field. The pseudocapacitive effect, which is led by the high specific area and the existence of ultrafine nanoparticles, takes on a feature of fast faradaic surface charge-transfer. Benefiting from the synergistic advantages of the heterostructures and the pseudocapacitive effect, the as-prepared graphene oxide/SnO₂/Co₃O₄ anode achieved a high reversible capacity of 461 mA h g^-1 after 80 cycles at a current density of 0.1 A g^-1. Additionally, at a high current density of 1 A g^-1, a high reversible capacity of 241 mA h g^-1 after 500 cycles is obtained. A full cell coupled by the as-prepared graphene oxide/SnO₂/Co₃O₄ anode and the Na₃V₂(PO₄)₃ cathode was also constructed, which exhibited a reversible capacity of 310.3 mA h g^-1 after 100 cycles at a current density of 1 A g^-1. This method of improving both interfacial and surface charge transfer may pave the way for the development of high performance sodium-ion batteries.